let x1: null = null;
let x2: undefined = undefined;

let x3: Symbol = Symbol(); // typeof x3 === "symbol"
let x4: bigint = 1n;
let x5: boolean = true;
let x6: number = 1;
let x7: string = 'abc'

const x8 = 'abc'  // 断言类型
// let x8: 'abc' = 'abc' // 字面类型
x7 = 'cde'

let x9 = new String('abc'); // .toString();
x9 = x8;
// (new String('abc')).toString();

class StringEx extends String { }
let x10 = new StringEx('abc');
x9 = x10;
// 字面对象类型
let x11: {
  a: string;
  b: number;
} = {
  a: "xx",
  b: 0,
}

let x12: object = x11;

// 空的类型或者空的接口
// let x13: {} = x10;
let x13: {} = {
  a: "xx",
  b: 0,
}



